<template>
	<ul class="list">
		<li 
			class="item"
			 v-for="item of letters" 
			 :key="item"
			 :ref="item"
			 @touchstart="handleTouchStart"
			 @touchmove="handleTouchMove"
			 @touchend="handleTouchEnd"
			 @click="handletterclick"
			 >{{item}}</li>
	</ul>
</template>

<script>
	export default {
		name: 'CityAlable',
		props: {
			cities: Object
		},
		computed: {
			letters () {
				const letters = []
				for (let i in this.cities) {
					letters.push(i)
				}
				return letters
			}
		},
		data () {
			return {
				touchStatus: false,
				startY: 0,
				timer: null
			}
		},
		updated () {
			this.startY =  this.$refs['A'][0].offsetTop
		},
		methods: {
			//兄弟组件通信
			handletterclick (e) {
				this.$emit('change',e.target.innerText )
			},
			//划过显示某块区域
			handleTouchStart () {
				this.touchStatus = true
			},
			handleTouchMove (e) {
				if (this.touchStatus) {
					if (this.timer){
						clearTimeout(this.timer)
					}
					this.timer = setTimeout(() => {
					const touchY = e.touches[0].clientY - 80
					const index = Math.floor((touchY - this.startY) /20)
					if (index >= 0 && index <= this.letters.length){
						this.$emit('change', this.letters[index])
					 }
					},16)
					//添加timer是优化性能
				}
			},
			handleTouchEnd () {
				this.touchStatus = false
			}
		}
	}
</script>

<style lang="stylus" scoped="scoped">
  @import '~styles/varibles.styl'
   .list
    position:absolute
    display:flex
    flex-direction: column
    justify-content:center
    top:80px
    right:0
    bottom:0
    width:20px
    .item
      color: $bgColor
      line-height:20px
      text-align:center
</style>